The present invention relates to a method for treating aqueous composition contaminants. In particular, the invention relates the treatment of aqueous compositions containing contaminating halogenated hydrocarbons, oxidized metal species, radionuclides, or mixtures thereof.
Halogenated hydrocarbons, particularly chlorinated hydrocarbons, are excellent solvents for many applications. They have low flammability and are fairly stable, both chemically and biologically. They are commonly used in industry as chemical carriers and solvents, paint removers and cleaners. The cleaning applications include metal degreasing, circuit board cleaning, metal parts cleaning and dry cleaning. Chlorinated solvents are also used as intermediates in chemical manufacturing and as carrier solvents for pesticides and herbicides.
Chlorinated hydrocarbons are very stable compounds and are relatively toxic at low levels. Groundwaters have become contaminated by chlorinated hydrocarbons from sources, such as disposal facilities, chemical spills and leaking underground storage tanks. Chlorinated solvents also may be released to the environment through the use, loss, or disposal of a neat liquid, or through the use or disposal of wash and rinse waters containing residual solvents. Chlorinated solvents are among the most common ground water contaminants be cause of their widespread use and stability.
Subsurface movement and dispersion of chlorinated solvents vary depending on whether the solvents are released as neat liquid or in dissolved form. If released in dissolved form, chlorinated solvent migration is governed largely by hydrogeological processes. The presence of solubilizing agents, such as soaps (from wash waters), counteracts natural soil sorption-retardation mechanisms and facilitates the migration of the dissolved solvents. If the chlorinated solvent is released as a neat liquid, the liquid solvent migrates downwardly through a soil column under the force of gravity. A portion of the solvent is retained in the soil pores. If sufficient solvent is present, however, the soil pores become saturated. Further, solvent then continues to move downwardly until it encounters a physical barrier or the water table. If the solvent encounters the water table, it spreads out until enough mass accumulates to overcome capillary forces. At this point, the greater density of the chlorinated solvent causes it to penetrate the surface of the water table and to travel downwardly by gravity until the mass of moving liquid is diminished by sorption or until it encounters an aquitard.
In recent years, groundwater contamination by chlorinated hydrocarbons from sources, such as disposal facilities, chemical spills and leaking underground storage tanks, has become a significant environmental problem. Many of these chlorinated hydrocarbons are highly toxic and classified as carcinogens or suspect carcinogens. Of particular concern are the chlorinated ethylenes, such as trichloroethylene (TCE), tetrachloroethylene, commonly known as perchloroethylene (PCE) and chlorinated ethanes, such as 1,1,1-trichloroethane (TCA), which have been used as degreasing agents in a variety of industrial applications. Although the use of chlorinated degreasing agents was severely curtailed in 1976, improper storage and uncontrolled disposal practices resulted in significant contamination to groundwater aquifers. Chlorinated solvents are highly mobile in soils and aquifers, due to their high water solubility (e.g., 1100 mg/L TCE at 25.degree. C.), and a need exists for removal treatments them from groundwater.
Pump-and-treat is a commonly applied treatment scheme for contaminated groundwater. The treatment usually involves withdrawing contaminated water from a well, volatilizing the contaminants in an air stripping tower, and adsorbing the vapor phase contaminants onto granular activated carbon (GAC). There are substantial limitations to pump-and-treat technology. The process is inefficient and some sites can require treatment for many decades.
It is known that chlorinated compounds can be degraded by reductive dechlorination, that is, replacement of chlorine substituents by hydrogen. Metallic elements, such as iron and zinc, have been used to degrade chlorinated organic compounds. Several systems have used iron metal to conduct reductive dechlorination of hydrocarbons in aqueous compositions. Gillham, in U.S. Pat. No. 5,266,213, discloses feeding contaminated groundwater through a trench containing an iron species. The process is conducted under strict exclusion of oxygen over a lengthy period of time. Large amounts of iron are needed for completion of the reactions. Additionally, it is difficult to introduce large volumes of solid reaction material, such as iron filings, into effective depths for in situ remediation.
Sivavec, in U.S. Pat. No. 5,447,639, teaches a method for enhanced remediation of aqueous solutions contaminated with chlorinated aliphatic hydrocarbons. The method comprises reacting reductively the chlorinated hydrocarbons with ferrous sulfide to generate innocuous byproducts, such as ethane, ethene, and chloride ion from chlorinated ethenes. Chlorinated aliphatic hydrocarbons, including trichloroethylene (TCE), tetrachloroethylene, and chlorinated ethanes such as 1,1,1-trichloroethane, are reduced to ethene, ethane, and chloride ion (Cl.sup.-1) when contacted with iron (II) sulfide under aerobic or anaerobic conditions. The reaction may proceed, in situ or ex situ, by an electron transfer mechanism at the mineral-water interface wherein ferrous ion (Fe.sup.+2) and/or sulfide in ferrous sulfide function as reducing agents and are oxidized to ferric ion (Fe.sup.+3) and sulfate (SO.sub.4.sup.-2), respectively.
According to U.S. Pat. No. 5,447,639, an effective amount of ferrous sulfide is admixed with a contaminated aqueous composition to generate ethane, ethene, and chloride ion. Granular ferrous sulfide may be filled into a pit, ditch, screened well or trench, and used to react with and degrade chlorinated aliphatic compounds in a migrating plume, such as groundwater aquifer and drainage runoff. In another aspect, a tower column is packed with ferrous sulfide. Industrial wastewater or pumped groundwater is then treated in the tower. Additionally, an inert filler, such as sand, gravel, pebbles, and the like, can be added to the ferrous sulfide to increase its hydraulic conductivity. Polymeric adsorbents, such as polyethylene, polypropylene, thermoplastic elastomers, and carbon-filled rubbers, can also be admixed with the granular ferrous sulfide.
Sivavec in U.S. Pat. No. 5,750,036 teaches a process for the reductive dehalogenation of halogenated solvents by contact with ferrous ion-modified clay minerals and iron (III)-containing soils, sediments or aquifer materials. Examples of ferrous ion sources include iron(II) sulfate heptahydrate, and the reductive dissociation product of magnetite (Fe.sub.3 O.sub.4) and oxalic acid.
According to U.S. Pat. No. 5,750,036, ferrous ion is introduced into clay minerals, clay bearing soils or sediments, iron(III) minerals and iron(III)-bearing soils, or sediments by a variety of methods. Examples of the methods include: (1) direct treatment of contaminated material with ferrous ion in aqueous solution; (2) dissolution of ferrous ion provided by the interaction of iron-bearing minerals with organic and inorganic reducing agents; (3) dissolution of ferrous ion resulting from iron metal corrosion; (4) dissolution of ferrous ion formed by electrolytic processes at iron electrodes; and (5) dissolution of ferrous ion produced by stimulation and growth of iron-reducing bacteria in iron-containing substrates such as soil sediment.
Lancy, in U.S. Pat. No. 3,294,680, discloses conditioning spent cooling water that has a toxic hexavalent chromium solution content. According to the method, a mass of hard metal sulfide granules is provided and cooling water is moved through the mass in contact with surface-reacting granules. The hexavalent chromium solution content is converted to trivalent chromium.
There remains a need to effectively treat aqueous compositions, particularly groundwater, that is contaminated with chlorinated hydrocarbons, oxidized metal species, radionuclides, or mixtures thereof.